Add RFC issue template

Structure taken from #771

.github/ISSUE_TEMPLATE/rfc.md

@@ -0,0 +1,24 @@
+---
+name: RFC
+about: Make a suggestion in a Request for Comments format to RustPython
+title: "[RFC] "
+labels: RFC
+assignees: ''
+
+---
+
+## Summary
+
+<!-- A quick overview of your suggestion -->
+
+## Detailed Explanation
+
+<!-- Elaborate on your suggestion in all its details -->
+
+## Drawbacks, Rationale, and Alternatives
+
+<!-- What drawbacks might this solution have? Why do you feel it is necessary? What other options might there be to solving this problem? -->
+
+## Unresolved Questions
+
+<!-- What would you like feedback on for fleshing out your suggestion? -->